1. Field of the Invention
This invention relates to improved photosensitive compositions and their use in photoelectrophoretic imaging methods and apparatus. More specifically, this invention is directed to the treatment of vanadyl phthalocyanine pigments with a combination of phenazine-type dyes and a mixture containing at least two polymers. Such treatment results in modification of the photoresponsiveness of the treated pigment and also thereby inhibits the injection of charge into the treated pigment from other sources.
2. Description of the Prior Art
As is generally recognized in the art, a photoelectrophoretic imaging system is one wherein electrically photosensitive particles dispersed in a carrier liquid are initially subjected to an electric field and either simultaneously or thereafter exposed to activating electromagnetic radiation conforming to an image pattern. Photoelectrophoretic imaging techniques may be adapted for the preparation of both monochromatic and polychromatic reproductions. A detailed disclosure of both these monochromatic and polychromatic photoelectrophoretic imaging systems can be found in U.S. Pat. Nos. 3,383,933; 3,384,488; 3,384,565 and 3,384,566 (all of which are hereby incorporated by reference in their entirety). In one of the preferred embodiments of the photoelectrophoretic imaging method described in the above patents, a layer of an imaging suspension comprising electrically photosensitive pigment particles in an insulating carrier liquid is arranged between an injecting electrode and a blocking electrode (at least one of the electrodes being at least partially transparent); the photosensitive dispersion subjected to an applied electric field; and thereafter exposed to activating electromagnetic radiation conforming to an image pattern. Typically, complementary images are formed on the opposing surfaces of the electrodes which are in contact with the dispersion of pigment particles. In a monochromatic system, pigment particles of only one color are required; however, particles of more than one shade of the same color may be utilized where one desires to provide the capability to produce a range of monochromatic colors. In a polychromatic system, images of more than one color, and preferably full color, may be formed by utilizing a plurality of differently colored pigment particles which ideally have spectral response curves which do not substantially overlap each other, thereby providing the necessary color separation. In the preferred photoelectrophoretic imaging system referred to hereinabove, the pigment particles correspond to the substractive colors yellow, cyan and magneta. The yellow pigment particles are primarily responsive to light within the blue region of the electromagnetic spectrum; the cyan particles are primarily photoresponsive to light within the red region of the electromagnetic spectrum; and the magenta particles are primarily responsive to light within the green region of the electromagnetic spectrum. Therefore, when a full color reproduction is projected upon a suspension containing these three pigments, the cyan particles will respond to that component of the image input corresponding to the color red, and upon being photoactivated will migrate from the electrode surface on which the image is to be formed thereby leaving behind the yellow and magenta pigment particles which together appear as red. Similarly, image input corresponding to green light will cause magenta particles to migrate and image input corresponding to blue light will cause the yellow particles to migrate. Where white light impinges upon the suspension containing the above three pigment particles, all such particles should migrate thereby leaving the surface of the image substantially devoid of pigment. The resulting image can thereafter be transferred to a receiving sheet, such as white paper, and thus the portions of the image which are deficient of pigment will appear as white in the finished copy. In order to obtain good color separation, it would be preferable that each pigment migrate only in response to activating electromagnetic radiation within its principle region of absorption.
Due to electrical interactions between the pigments and other unknown factors, photostimulated particle migration is often incomplete resulting in traces of the "subtracted" pigment remaining at the injecting electrode thereby imparting undesired color to the image formed on this electrode.
As is discussed in the patents previously incorporated by reference, the pigment particles used in photoelectrophoretic imaging systems are initially charged and caused to migrate to the surface of one of two opposing electrodes in response to an electric field established between these electrodes. Upon absorption of light within its principal region of photoresponse, these pigments, it is theorized, generate hole-electron pairs and, depending upon the relative mobility of these charge carriers in the pigment, either one or both of these charge carriers are injected into the liquid carrier medium. Upon the injection of only one species of carrier into the medium, the particle will thereby acquire a net charge which preferably will be identical in sign to the polarity of charge of the electrode to which it had previously migrated. This similarity in net charge will cause the pigment particle to be repelled by this formerly attractive electrode resulting in its migration to the surface of the opposing electrode where it forms a complementary image. It will be appreciated that if the above theoretical explanation is correct, the injection of both species of charge carrier into the liquid carrier medium will result in a failure of the photoactivated particle to migrate and thus in failure to generate the desired image. Moreover, in the event of indiscriminate injection of charge carriers from the photoactivated pigment into the liquid carrier medium and the subsequent transfer of such carriers to a non-photoactivated particle, the non-photoactivated pigment particle will migrate just as if it had absorbed the imaging energy. This migration of non-photoactivated particles will seriously impair color separation in the desired reproduction.
It thus appears that in order for good color separation to be maintained and faithful reproduction of an original to be achieved, it is necessary to maintain selective electrophotographic response of the pigments to their colors of primary absorption. It is also apparent that this can only be achieved by preventing indiscrimate injection of charge carriers from photoactivated pigments into the liquid carrier medium.
The prior art contains frequent reference to various treatment of photoelectrophoretic pigments with diverse materials in order to modify or enhance the electrophotographic response of such pigments. The literature discloses, for example, (a) the adsorption of donor and acceptor molecules on pigments utilized in photoelectrophoretic imaging, (b) the inclusion of such electrically active materials in the insulating liquid carrier containing such pigment particles, or (c) the application of these electrically active materials to one of the electrodes used in confining the pigment dispersion. All of the above treatments are said to result in charge transfer complex formation between the pigments and these electrically active materials, thereby facilitating injection of electrons from photoactivated pigment particles into the surrounding medium, U.S. application Ser. No. 566,846, filed July 21, 1966, now abandoned; published in Japan on Mar. 20, 1970, application Ser. No. 463,667, filed July 20, 1967.
Photoactive polymeric materials have also been disclosed as effective in modification of the electrophoretic response of pigment particles used in photoelectrophoretic imaging systems, U.S. application Ser. No. 863,507, filed Oct. 3, 969, published in Holland on Apr. 6, 1971 as application Ser. No. 70.14614. Poly(N-vinylcarbazone) is disclosed in this Dutch patent as useful in the agglomeration and/or encapsulation of photomigratory pigment particles thereby enhancing the electrophotographic response of these particles to imaging energies.
The prior art systems referred to hereinabove are primarily concerned with the "enhancement" in the photoresponse characteristics of the pigments involved in such treatment. Under certain conditions, however, it may prove desirable to reduce the degree of photoresponsiveness of the pigment particles; as in the case where such photoactivated pigments tend to interact with non-activated pigment particles, or are substantially more photoresponsive (in terms of migrational efficiency and speed) than other photoactivated pigments also present in the imaging suspension.
Accordingly, it is the object of this invention to provide a method for treatment of pigment particles designed to inhibit the photoresponsiveness of materials in a photoelectrophoretic environment.
More specifically, it is the object of this invention to provide a photosensitive composition which is both inhibited from interaction with non-photoactivated pigments and also relative resistant to acceptance of charge from other materials used in conjunction therewith.
It is another object of this invention to provide a method for balancing the photoresponsiveness of a pigment so as to reduce its photomigratory response in relation to other pigments used in conjunction therewith.
Additional objects of this invention include the preparation of photosensitive composition according to the above methods and the use of such compositions in photostimulated particle migration imaging systems.